1. Field of the Invention
The present invention relates to a stepping motor.
2. Description of Related Art
One previously proposed stepping motor includes a casing, which houses a stator and a magnet rotor. The stator has a plurality of salient poles, which extend radially inwardly. The magnet rotor is rotatably arranged inside the stator in such a manner that a predetermined space is provided between each salient pole and the magnet rotor. The stator is immovably secured in the casing, and the magnet rotor is rotatably supported in the casing (see, for example, Japanese Unexamined Patent Publication No. 2003-111379).
In the case of the above stepping motor, the stator is press fitted into a recess of the casing to immovably secure the stator in the casing. In order to facilitate the press fitting operation of the stator into the casing, an inlet opening end of the recess is chamfered. That is, each lateral side of the inlet opening end of the casing is provided with a single slant surface, so that a width of the recess is progressively increased from a bottom side of the recess toward the inlet opening end side of the recess. In this way, the positioning of the stator into the recess of the casing is eased.
In the process of press fitting the stator into the recess of the casing, a corner of the stator contacts the slant surface at the chamfered inlet opening end. Thereafter, the stator is further pressed toward the bottom of the recess while expanding the width of the recess.
At this time, the casing is likely scraped by each corresponding corner of the stator, so that scraped debris (i.e., foreign object) is generated from the casing. When the foreign object is left in the stepping motor, the foreign object may penetrate into a space of a torque transmission mechanism (e.g., a gear train G) or into a space between the magnet rotor and the salient pole of the stator in the stepping motor. Such a foreign object may possibly limit smooth movement of the stepping motor.
To obviate the above disadvantage, the foreign object must be removed after the press fitting of the stator in the manufacturing of the stepping motor. This disadvantageously increases the number of manufacturing steps of the stepping motor and thereby causes an increase in the manufacturing costs.